MRAM Cell and Method for Writing to the MRAM Cell using a Thermally Assisted Write Operation with a Reduced Field Current

ABSTRACT

The present disclosure concerns a method for writing to a MRAM cell comprising a magnetic tunnel junction formed from a storage layer having a storage magnetization; a reference layer having a reference magnetization; and a tunnel barrier layer included between the sense and storage layers; and a current line electrically connected to said magnetic tunnel junction; the method comprising: passing a heating current in the magnetic tunnel junction for heating the magnetic tunnel junction; passing a field current for switching the storage magnetization in a written direction in accordance with the polarity of the field current. The magnitude of the heating current is such that it acts as a spin polarized current and can adjust the storage magnetization; and the polarity of the heating current is such as to adjust the storage magnetization substantially towards said written direction.

FIELD

The present disclosure concerns a random access memory (MRAM) cell and amethod for writing to the MRAM cell using a thermally assisted writeoperation with a reduced field current.

BACKGROUND

Random access memory (MRAM) cells using a thermally assisted writeoperation usually comprise a magnetic tunnel junction formed from areference layer having a fixed magnetization, a storage layer having amagnetization that can be switched and a tunnel barrier between thereference and storage layers. The MRAM cell further comprises anantiferromagnetic layer exchange-coupling the magnetization of thestorage layer. Such MRAM cells are characterized by a considerablyimproved thermal stability of the storage layer due to theexchange-coupling of the antiferromagnetic layer. An improved writingselectivity of such MRAM cells is also achieved due to the selectiveheating of the memory cell to be written in comparison with theneighboring memory cells remaining at ambient temperature. The MRAM cellis written using a field current passing in a field line such as togenerate a magnetic field adapted to switch the magnetization of thestorage layer when the memory cell is heated. The magnitude of the fieldcurrent can be however too high for low power applications.

SUMMARY

The present disclosure concerns a method for writing to a MRAM cellcomprising: a magnetic tunnel junction comprising a storage layer havinga storage magnetization that can be adjusted when the magnetic tunneljunction is heated to a high temperature threshold and fixed when themagnetic tunnel junction is cooled to a low temperature threshold; areference layer having a fixed reference magnetization; and a tunnelbarrier layer included between the sense and storage layers; and acurrent line electrically connected to said magnetic tunnel junction;the method comprising:

passing a heating current in the magnetic tunnel junction via thecurrent line for heating the magnetic tunnel junction;

once magnetic tunnel junction has reached the high temperaturethreshold, passing a field current such as to switch the storagemagnetization in a written direction substantially parallel orantiparallel relative to the reference magnetization, in accordance withthe polarity of the field current;

the magnitude of the heating current is such that it acts as a spinpolarized current and exerts an adjusting spin transfer on the storagemagnetization;

the polarity of the heating current being such as to adjust the storagemagnetization substantially towards said written direction.

In an embodiment, the MRAM cell can further comprise a bipolartransistor in electrical connection with one end of the magnetic tunneljunction, the bipolar transistor being arranged for controlling thepassing of the heating current in the magnetic tunnel junction and thepolarity of the heating current.

In another embodiment, the field current can be passed in the currentline. Alternatively, the MRAM cell can comprise a field line, and thefield current can be passed in the field line.

In yet another embodiment, the MRAM cell can further comprise a storageantiferromagnetic layer exchange coupling the storage layer and pinningthe storage magnetization when the magnetic tunnel junction is at thelow temperature threshold and freeing the storage magnetization when themagnetic tunnel junction is at the high temperature threshold.

The MRAM cell and the method for writing to the MRAM cell disclosedherein allows for combining the heating current acting as a spinpolarized current at the high current threshold, with the field currentfor switching the storage magnetization. The field current used forswitching the storage magnetization can be reduced compared to aconventional MRAM cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with the aid of the descriptionof an embodiment given by way of example and illustrated by the figures,in which:

FIG. 1 shows a view of a random access memory (MRAM) cell comprising amagnetic tunnel junction, a select transistor, a current line forpassing a heating current, and a field line for passing a field current,according to an embodiment;

FIG. 2 illustrates the MRAM cell according to another embodiment;

FIG. 3 illustrates the MRAM cell according to another embodiment,wherein the heating current and the field current are passing in thecurrent line; and

FIG. 4 represents the magnetic tunnel junction comprising a syntheticstorage layer, according to an embodiment.

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS

FIG. 1 shows a random access memory (MRAM) cell 1 according to anembodiment. The MRAM cell 1 comprises a storage layer 23 having astorage magnetization 230 that can be adjusted when the magnetic tunneljunction 2 is heated to a high temperature threshold and fixed when themagnetic tunnel junction 2 is cooled to a low temperature threshold. Themagnetic tunnel junction 2 further comprises a reference layer 21 havinga fixed reference magnetization 210, and a tunnel barrier layer 22included between the sense and storage layers 21, 23. The MRAM furthercomprises a current line 3 electrically connected to one end of themagnetic tunnel junction 2 arranged to pass a heating current 31. TheMRAM can further comprise a select transistor 8 electrically connectedto the other end of the magnetic tunnel junction 2. A control currentline, or word line (not represented), can be used to control the openingand the closing of the select transistor 8 in order to address the MRAMcell 1 individually. The select transistor 8, for example, can comprisea CMOS transistor. In the example of FIG. 1, the MRAM cell 1 furthercomprises a field line 4 arranged at said one end of the magnetic tunneljunction 2 and substantially perpendicular to the current line 3 andadapted for passing a field current 41. In FIG. 1, the field line andthe field current 41 are represented perpendicular to the page.

In an embodiment, a write operation for writing to the MRAM cell 1comprises:

passing the heating current 31 in the magnetic tunnel junction 2 via thecurrent line 3 for heating the magnetic tunnel junction 2;

once magnetic tunnel junction 2 has reached the high temperaturethreshold, passing the field current 41 such as to switch the storagemagnetization 230 in a written direction;

cooling the magnetic tunnel junction 2 to the low temperature thresholdsuch as to pin the storage magnetization 230 in the written direction.

The field current 41 can be passed in the field line 4 such as togenerate a write magnetic field 42 having a direction that depends onthe sense, or polarity, of the field current 41. In FIG. 1( a), thefield current 41 is shown with a first field current polarity, hereentering the page, such that the write magnetic field 42 switches thestorage magnetization 230 in the written direction that is substantiallyparallel to the reference magnetization 210. The parallel arrangementbetween the storage magnetization 230 and the reference magnetization210 yields a low junction resistance R (or level state “0”). In FIG. 1(b), the field current 41 is shown with a second field current polarity,here exiting the page, such that the write magnetic field 42 switchesthe storage magnetization 230 in the written direction that issubstantially antiparallel to the reference magnetization 210. Theantiparallel arrangement between the storage magnetization 230 and thereference magnetization 210 yields a high junction resistance R (orlevel state “1”).

Passing the heating current 31 in the magnetic tunnel junction 2 can beachieved by setting the select transistor 8 in its conducting mode (ON).When the magnetic tunnel junction 2 has reached the high temperaturethreshold, the storage magnetization 230 can be freely aligned and thusswitched in the write magnetic field 42. The heating current 31 can thenbe turned off by setting the select transistor 8 in the cutoff mode(OFF) and/or by removing the transistor's source-drain bias. The fieldcurrent 41 can be maintained during the cooling of the magnetic tunneljunction 2, and then switched off, when the magnetic tunnel junction 2has reached the low temperature threshold wherein the storagemagnetization 230 is frozen in the written state.

In an embodiment, the magnetic tunnel junction 2 comprises aantiferromagnetic reference layer 24 exchange-coupling the referencelayer 21 such as to pin the reference magnetization 210 below areference critical temperature T_(C1) of the antiferromagnetic referencelayer 24. The magnetic tunnel junction 2 can further comprise aantiferromagnetic storage layer (show in FIG. 1 by numeral 25) having astorage critical temperature T_(C2) and exchange-coupling the storagelayer 23. The storage antiferromagnetic layer is arranged to pin thestorage magnetization 230 at the low temperature threshold, below thestorage critical temperature T_(C2), and to free the storagemagnetization 230 at the high temperature threshold, at or above thestorage critical temperature T_(C2). The storage critical temperatureT_(C2) should be lower than the reference critical temperature T_(C1)such that, at the high temperature threshold, the referencemagnetization 210 remains pinned by the antiferromagnetic referencelayer 24.

The magnitude of the heating current 31 required for heating themagnetic tunnel junction at the high temperature threshold is typicallybelow the magnitude needed for obtaining a spin transfer torque (STT)effect. In the case the reference critical temperature T_(C1) of theantiferromagnetic reference layer 24 is high enough, the magnitude ofthe heating current 31 required to heat the magnetic tunnel junction tothe high temperature threshold can be such that the heating current 31generates the STT effect. The STT effect so generated can be such as toorient the storage magnetization 230 is a direction that is differentthan the one of the write magnetic field 42 during the write operation.The STT effect can thus produce unwanted effects on the applied writemagnetic field 42, such as write magnetic field asymmetry, broadening ofthe write magnetic field distribution, or even writing errors.

In an embodiment, the heating current 31 is passed with a magnitudecorresponding to a high current threshold that is sufficient for theheating current 31 to act as a spin polarized current. The heatingcurrent 31 becomes polarized when passing through the reference layer 21or through a possible polarizing layer (not shown), according to theflow direction, or polarity, of the heating current 31. At the currentthreshold, the storage magnetization 230 can then be adjusted bytransfer of the angular spin moment between the spin-polarized carriers(electrons) of the heating current 31 and the storage magnetization 230.This transfer of the angular spin is also known under the expression“spin transfer torque (STT)”.

According to the polarity of the heating current 31, the spins of theelectrons penetrating into the storage layer 23 are in majority orientedalong the reference magnetization 210 or a magnetization of the possiblepolarizing layer. More particularly, the polarity of the heating current31 can be selected such as to exerts an adjusting spin transfer on thestorage magnetization 230 substantially in the written direction, i.e.,such that the heating current 31 adjusts the storage magnetization 230substantially in the same direction as the direction the write magneticfield 42 switches the storage magnetization 230. This is illustrated inFIG. 1 where FIG. 1( a) shows the heating current 31 having a firstheating current polarity, here flowing from the current line 3 towardsthe select transistor 8, such as to align the storage magnetization 230in the same direction (written direction) as the one provided by thewrite magnetic field 42 generated by the field current 41 having thefirst field current polarity. In FIG. 1( b), the heating current 31 isrepresented having a second heating current polarity opposite to thefirst heating current polarity, here flowing from the select transistor8 towards the current line 3, such as to align the storage magnetization230 in the same direction (written direction) as the one provided by thewrite magnetic field 42 generated by the field current 41 having thesecond field current polarity. In contrast with conventional MRAM cellsusing a monopolar select transistor, the select transistor 8 is bipolarallowing for changing the polarity of the heating current 31.

In yet another embodiment, the storage layer 23 can be a syntheticstorage layer comprising a first ferromagnetic layer 231 on the side ofthe tunnel barrier layer 22 and having a first ferromagneticmagnetization 232, a second ferromagnetic layer 233 having a secondferromagnetic magnetization 234, and a non-magnetic coupling layer 235separating the first and second ferromagnetic layers 231, 233. Themagnetic tunnel junction 2 comprising such a synthetic storage layer 23is represented in FIG. 4. Passing a field current 41 switches the firstand second ferromagnetic magnetization 232, 234 relative to thereference magnetization 210, such that the first storage magnetization232 is in the written direction. The storage magnetization (not shown inFIG. 4) corresponds to the vectorial sum of the first and secondferromagnetic magnetizations 232, 234.

FIG. 2 shows the MRAM cell 1 according to another embodiment. The MRAMcell 1 of FIG. 2 is substantially the same as the one represented inFIG. 1 but having the field line 4 being arranged at said other end ofthe magnetic tunnel junction 2, i.e., on the side of the selecttransistor 8. Although not shown in FIG. 2, the magnetic tunnel junction2 can also comprise the antiferromagnetic storage layer described in theexample of FIG. 1. In the configuration of FIG. 2, the select transistor8 is electrically connected to the other end of the magnetic tunneljunction 2 via a conductive strap 7. FIG. 2( a) shows the field current41 passing in the field line 4 with the first field current polarity andthe heating current 31 having the first heating current polarity. Boththe field current 41 and the heating current 31 aligning the storagemagnetization 230 in the written direction, here in the written levelstate “0”. FIG. 2( b) shows the field current 41 passing in the fieldline 4 with the second field current polarity and the heating current 31having the second heating current polarity. Both the field current 41and the heating current 31 aligning the storage magnetization 230 in thewritten direction, here in the written level state “1”.

FIG. 3 shows the MRAM cell 1 according to yet another embodiment,wherein the MRAM cell 1 only comprises the current line 3 for passingthe heating current 31 and the field current 41. Although not shown inFIG. 3, the magnetic tunnel junction 2 can also comprise theantiferromagnetic storage layer described in the example of FIG. 1.FIGS. 3( a) and 3(b) show both the field current 41 and the heatingcurrent 31 passing in the current line 3. In FIG. 3( a), the fieldcurrent 41 flows with the first field current polarity and the heatingcurrent 31 flows with the first heating current polarity, such as toalign the storage magnetization 230 in the written direction, here inthe written level state “0”. In FIG. 3( b), the field current 41 flowswith the second field current polarity and the heating current 31 flowswith the second heating current polarity, such as to align the storagemagnetization 230 in the written direction, here in the written levelstate “1”. In the configuration of FIG. 3, the current line 3 fulfillsthe function of a bit line by passing the heating current 31 and of afield line by passing the field current 41.

The method disclosed herein allows for combining the heating currentacting as a spin polarized current at the high current threshold, withthe field current 41 for switching the storage magnetization 230. Inother words, when the heating current is passed at the high currentthreshold and with a suitable polarity, it can assist the magnetic field42 generated by the field current 41 in switching the storagemagnetization 230. An advantage of passing the heating current acting atthe high current threshold is that the write magnetic field 42, and thusthe field current 41, can be reduced compared to a conventional MRAMcell.

A magnetic memory device (not shown) can be formed by assembling anarray comprising a plurality of the MRAM cell 1. The array of MRAM cells1 can be disposed within a device package (not shown). When forming themagnetic memory device, the magnetic tunnel junction 2 of each MRAM cell1 can be connected on the side of the storage layer 23 to the currentline 3 and on the opposite side to the word line (not shown). The wordline is preferably placed perpendicularly with reference to the currentline 3.

REFERENCE NUMBERS

-   1 magnetic random access memory (MRAM) cell-   2 magnetic tunnel junction-   21 reference layer-   210 reference magnetization-   22 tunnel barrier layer-   23 storage layer-   230 storage magnetization-   231 first ferromagnetic layer-   232 first ferromagnetic magnetization-   233 second ferromagnetic layer-   234 second ferromagnetic magnetization-   235 non-magnetic coupling layer-   24 antiferromagnetic reference layer-   25 antiferromagnetic storage layer-   3 current line-   31 heating current-   4 field line-   41 field current-   42 write magnetic field-   7 strap-   8 select transistor

1. Method for writing to a random access memory (MRAM) cell using athermally assisted write operation, the MRAM cell comprising: a magnetictunnel junction comprising a storage layer having a storagemagnetization that can be adjusted when the magnetic tunnel junction isheated to a high temperature threshold and fixed when the magnetictunnel junction is cooled to a low temperature threshold; a referencelayer having a fixed reference magnetization; and a tunnel barrier layerincluded between the sense and storage layers; and a current lineelectrically connected to said magnetic tunnel junction; the methodcomprising: passing a heating current in the magnetic tunnel junctionvia the current line for heating the magnetic tunnel junction; oncemagnetic tunnel junction has reached the high temperature threshold,passing a field current which generates a write magnetic field whichswitches the storage magnetization in a written direction substantiallyparallel or antiparallel relative to the reference magnetization, inaccordance with the polarity of the field current; the magnitude of theheating current being such that it acts as a spin polarized current andexerts an adjusting spin transfer on the storage magnetization; and thepolarity of the heating current being such as to exert an adjusting spintransfer on the storage magnetization substantially towards said writtendirection.
 2. Method according to claim 1, wherein the MRAM cell furthercomprises a bipolar transistor in electrical connection with one end ofthe magnetic tunnel junction, the bipolar transistor being arranged forcontrolling the passing of the heating current in the magnetic tunneljunction and the polarity of the heating current.
 3. Method according toclaim 1, wherein the field current is passed in the current line. 4.Method according to claim 1, wherein the MRAM cell further comprises afield line and wherein the field current is passed in the field line. 5.Method according to claim 1, wherein the MRAM cell further comprises astorage antiferromagnetic layer exchange coupling the storage layer andpinning the storage magnetization when the magnetic tunnel junction isat the low temperature threshold and freeing the storage magnetizationwhen the magnetic tunnel junction is at the high temperature threshold.6. Method according to claim 1, wherein the storage layer comprises afirst ferromagnetic layer on the side of the tunnel barrier layer andhaving a first ferromagnetic magnetization, a second ferromagnetic layerhaving a second ferromagnetic magnetization, and a non-magnetic couplinglayer separating the first and second ferromagnetic layers, and whereinsaid passing a field current switches the first and second ferromagneticmagnetization relative to the reference magnetization, such that thefirst storage magnetization is in the written direction.